Foam of course, is a most undesirable by-product in most industries since it does have a direct and drastic effect upon production efficiency and accordingly the economics of a system or process. In some systems, foam can be tolerated to a certain extent; however, in most systems such as the pulp and paper producing processes, the quality of the product is dependent upon the control of foam.
At present there are a number of different chemical pulping processes finding extensive use in the industry. However, by far the most widespread are the alkaline pulping processes commonly referred to as the "soda" and the "kraft" processes.
Kraft pulp, also referred to as sulfate pulp, represents one type of chemical pulp. It is, perhaps, the most important of the chemical pulps, as indicated by the increased number of kraft pulp mill installations and by the fact that many of the mills now under construction are planning to utilize the kraft process. The reasons for the popularity of this process are many among which can be mentioned the strength of the resultant pulp, the varieties of wood that lend themselves to this process, and the excellent degree of chemical recovery of cooking liquors which is possible.
Because kraft provides the strongest pulp, its primary use is where strength is an important consideration. Kraft is used for making wrapping paper, grocery bags, heavy duty shipping bags, cartons, etc. It is also mixed with other types of pulp where added strength is required.
The kraft process is an alkaline process with the active cooking chemicals being caustic soda and sodium sulfide. The presence of the sodium sulfide is what makes the kraft process different from the soda process.
In the kraft process, measured quantities of standard sized chips are directed to a digester which is a tall cylindrical pressure vessel with a conical bottom. Briefly stated, in addition to the wood chips, white liquor (which is the cooking liquor, the active chemical ingredients of which are caustic soda and sodium sulfide) and steam are charged to the digester. Through the action of the cooking liquor and steam on the chips, the lignin binder is dissolved, freeing the cellulose fibers, thus the chips are converted into a brown colored kraft pulp. The cooking is performed normally at pressures varying from 80 - 110 psi. Although the active chemicals in the cooking liquor (white liquor) are caustic soda and sodium sulfide, the liquor will also contain some sodium carbonate and some sodium sulfate. The kraft process is sometimes called "sulfate" because sodium sulfate (saltcake) is the makeup chemical for converting the black liquor to white liquor.
In the cooking process in the digester, the white liquor becomes black as it is spent in cooking. After the cook, the pulp and black liquor are blown out of the digester to a blow pit and directed over a knot screen and then to vacuum washers, at which point the black liquor is separated from the pulp. The pulp is then sent to the bleach plant, or if unbleached kraft is to be used, the pulp is directed to the paper mill.
The pulp washing, which accomplishes the removal of the black liquor from the pulp, is the point at which brown stock defoamers and drainage aids are required. A screening operation usually precedes the washing stage to remove any knots which may be ejected from the digester into the blow tank. The washing takes place in a group of washers connected in series and is countercurrent.
Weak liquor removed from the pulp on the last washer is sprayed on the pulp on the previous washer. This continues back to the first washer. By using this process, the pulp is always washed by a liquor of lower solids content that the pulp itself contains. This permits removal of solids from the pulp with minimum fresh water requirements.
As earlier stated, although the soda and kraft processes possess distinct advantages with respect to the reclaiming and reuse of the spent chemicals, the processes do possess inherent disadvantages due to the foaming problems which are encountered at various steps in the process. The most troublesome areas are at the pulp washing, screening and knotting stages of the operation. During these stages a considerable amount of troublesome foam is generally formed. Likewise, when the resulting pulp is being washed in the brown stock washers to remove residual chemicals commonly referred to as black liquor, a significant foam problem is encountered. A black liquor contains from 13 to 20% by weight of dissolved solids and has a pH in the range of from about 11 to 13. Because of the constitution of the black liquor, that is, its resin content, its dissolved solids content and the ph of the system, there is a definite foaming potential, which if allowed to occur, affects the entire system deleteriously.
Foam is equally encountered after the pulp has left the brown stock washers and has traveled to the screenroom where the pulp is again diluted with water and passed through the various screening operations. Thiss operation allows the satisfactory fibers to pass through while the clumps of unpulped fibers, knots or other foreign material is retained on the screen. In addition, foam becomes quie significant in the screenroom where the diluted pulp containing a small residual amount of black liquor is subjected to violent agitation by the screen. The black liquor removed during the screening operation is normally used as the dilution water in in the various stages of the brown stock washers; therefore, since the diluted black liquor still contains a minor amount of solids, a foaming potential exists.
The foaming problems encountered at various stages of the operation are dependent upon a number of factors of which may be mentioned the type wood used in the pulp making process, the conditions, e.g., temperature and the extent of agitation, the amount and type of undissolved solids dispersed in the liquor and the dissolved materials in the liquor. It is common in some pulping processes to find that foaming is not a particular problem at one stage but that it does become such at a later stage as when the liquor or effluent of one stage is returned to the system as a washing or diluting agent at another stage of the operation.
Since the foaming problems do have a significant effect on the efficiency of the pulp producing process and since the economics of the pulp producing system itself are greatly affected, many defoaming compositions have been proposed for use during the stages of operation in question. The various defoamers recommended for use, although possessing distinct advantages in some aspects, also possess attendant disadvantages. For example, a formulated defoamer or antifoaming agent may be satisfactory with one particular type wood such as spruce or fir, but may not be at all successful in retarding foam where the fibers are derived from hemlock. Moreover, it is not unusual to find differences when the same type wood is utilized to produce a pulp. The conditions of the operation in one instance may be such as to render a defoaming wholly inoperable while at the same time this same defoamer may be perfectly successful in another similar process. The differences in the respective operations can be attributed to the relative differences in temperature, the type water used and the respective agitation and/or aeration times utilized.
Another important consideration is the cost involved. Although a particular defoamer may have successfully inhibited the foaming potential of a particular system, the feed rate and the initial cost of a defoamer or drainage aid may be such as to make its use prohibitive. Accordingly, it can be appreciated that when all of the factors are considered, it is extremely difficult to formulate a particular composition which will not only perform the function desired but which will also be operable at the specific feed rates demanded by the mills. With the foregoing in mind, the present inventors embarked upon a comprehensive study in an attempt to produce a defoamer or antifoaming agent which would fulfill as many of the prerequisites as possible. The primary objective of course, was to obtain a defoaming composition which would not only satisfy the demands of all of the systems wherein the alkaline pulp producing process is utilized, but also one which would be economically desirable to the mill.